This invention relates generally to normalization of head-mounted display brightness to ambient light level brightness. More specifically, the invention describes a method and apparatus for using ambient light to backlight a display panel in a head-mounted display unit.
Head-mounted displays utilize projection techniques or other methods to place information or symbols in or very near the user's field of vision for fast reference or targeting.
Generally, the head-mounted display system is utilized when the user is attempting to monitor both the information on the display, and simultaneously remain vigilant about his environment.
The ambient light levels of the scene viewed by the user may vary as much as 2000 to 1 in brightness. Consequently, for the user to be able to effectively view the image on the display and the scene at the same time, the brightness level of the scene and the image on the display when they reach the eye must be similar, or the user's eyes will not register both. Thus display brightness must be constantly adjusted to compensate for changes in ambient brightness.
Adjustment for the backlight in the past may have been manual, and thus under the user's control, or may have been done by a microcontroller or similar device in conjunction with a light sensor. The disadvantage of the former is that the user must have used some of his/her concentration to adjust the light level of the display image which may in some cases be extremely inconvenient. An electronically controlled backlight does not disturb the user, but requires some form of control algorithm, which has proven to be a difficult control problem.
Use of a backlight for the display has other difficulties. It adds weight to the display regardless of how it is controlled; heating of the user's head is a problem, as is power usage. In many cases, in fact, it has not been possible to mount a light on the head-mounting to compensate for the large range of ambient light changes, without unacceptable power usage and/or excessive weight and heat dissipation on the helmet.
Movement of components and lighting sources off from the head-mounting and transmitting light to the head-mounting via fiber optic cables is another option, but typically this solution restricts movement of the users head and the cables are subject to breaks and misconnections.